A Point Charge Model for the Ferroelectric Transition in Ammonium Sulphate

In ammonium sulphate the distortion of the SO42− ion, which has almost tetrahedral symmetry in its paraelectric phase, occurs in a narrow region near the ferroelectric transition temperature Tc = 223 °K. A point charge model has been used to compute the spontaneous polarization (P) due to NH and SO ions in the crystal. The computed value of P (0.42 μC/cm2) agrees well with the experimentally reported ones. The nature of phase transition has also been discussed.